1. Field of the Invention
The invention relates to a liquid crystal display panel, and more particularly to an exposure mask that compensates light intensity deviation of an exposer.
2. Discussion of the Related Art
Generally, liquid crystal display devices display pictures by controlling the light transmissivity of liquid crystal having a dielectric anisotropy using an electric field. The liquid crystal display devices include a liquid crystal display panel that has a plurality of liquid crystal cells arranged in a matrix configuration for displaying pictures, and a drive circuit for driving the liquid crystal display panel. Active matrix type liquid crystal displays, which independently drive liquid crystal cells using a thin film transistor, are widely used for televisions as well as display devices for personal computers.
The liquid crystal display panel includes a thin film transistor substrate and a color filter array substrate, which are opposite to each other; with liquid crystal provided between the two substrates; and a spacer to maintain a cell gap between the substrates.
The thin film transistor substrate includes gate and data lines; a thin film transistor formed near the crossing of the gate and the data lines; a pixel electrode at each of the liquid crystal cells connected to the thin film transistor; and an alignment film formed thereover.
The color filter substrate includes color filters corresponding to the liquid crystal cells; a black matrix between the color filters to reflect external light; a common electrode to commonly supply a reference voltage to the liquid crystal cells; and an alignment film formed thereover.
The thin film transistor substrate and the color filter substrate are made separately, bonded together, and then liquid crystal is provided between the two substrates and sealed, to form the liquid crystal display panel.
A plurality of mask processes are required to form the patterns of the thin film transistor substrate and the color filter substrate. Each mask process includes a thin film deposition process, a cleansing process, a photolithography process, an etching process, a photo-resist exfoliation process and an inspection process. When the substrate is larger than the effective area of the exposer used in the photolithography process, a stitch (divided) exposure method is employed. The stitch exposure method includes exposing the substrate in sections.
FIG. 1 illustrates a stitch exposure method according to a related prior art. Referring to FIG. 1, a substrate 20 has a thin film (a metal layer, an insulating film, a semiconductor layer or the like, not shown) for patterning, and a photo-resist formed on the thin film. By exposing the photo-resist to light through a mask 10, a photo-resist pattern is formed corresponding to a pattern of the mask 10 during the exposure process. Because the substrate 20 is larger than the mask 10, the exposure process is repeated to form the photo-resist pattern shown in FIG. 1, while moving the mask 10. The unit of one exposure process using the mask is called a shot, and the exposed area of the substrate corresponding to one shot is the shot area. For example, the photo-resist pattern on the substrate 20, shown in FIG. 1, has four shot areas A to D. In other words, while moving the mask 10, the photo-resist pattern of each shot area, A to D, is sequentially exposed.
Each shot area is exposed to light by an exposer that scans in a vertical direction of the substrate 20. Generally, the exposer is such that the light intensity (luminous intensity) is relatively high at the middle part of the shot area and is low at the left and/or right side of the shot area. An exemplary relationship between light intensity of an exposer and location of a shot area is illustrated in FIG. 1. Furthermore, the light intensity decreases in a different ratio between the left and right sides of the shot area, which is also illustrated in FIG. 1. The difference in light intensity between the left and right sides of the shot area causes the pattern formed on the left side and the pattern formed on the right side to have different critical dimensions CD, i.e., a deviation in the pattern's thickness and location. Because there is a CD deviation between the patterns formed at the boundary between adjacent shot areas, a stitch stain is introduced in the liquid crystal display panel.
For example, the pattern formed in the middle part of the shot area has a CD of P, and due to the difference in light exposure, the pattern formed at the left side of each of the shot areas A to D has a CD of P+2a, and the pattern formed at the right side has a CD of P+a. Generally, the light intensity at the right side, which is greater than the light intensity at the left side, causes the pattern formed on the right side of the shot area to have a greater CD than the pattern formed on the left side of the shot area. Accordingly, a boundary area between adjacent shot areas A to D is formed by a left pattern having a CD of P+2a and a right pattern having a CD of P+a, resulting in a CD deviation between the shot areas. In other words, the left and right patterns, which have a CD deviation ratio of 2a:a, are adjacent to each other in the boundary area of the shot areas A to D. The stitch stain is generated in the liquid crystal display panel due to the CD deviation in the boundary area between the shot areas A to D.
In order to solve this problem, a method has been proposed wherein patterns are formed to be divided at both sides of the mask and the patterns are overlapped to be composed in the adjacent shot areas, thereby reducing the CD deviation. However, because of the difference in light intensity between the left and right sides of the exposer, the CD deviation between the patterns formed at the left and right sides of the mask remains, and the stitch stain problem still exists.